feat(graph): add graph tab for whole-run data

Provides a static line chart plotting multi-sensor temperature values
over time when a session file is loaded.

tests/test_session_controller.py

@@ -53,6 +53,26 @@ def test_pause_stop_timer(controller):
     controller.stop()
     assert controller.frameIndex == 0
 
+def test_graph_data_empty_before_load(controller):
+    assert controller.graphSensorNames == ""
+    assert controller.graphSeriesJson == "[]"
+
+def test_graph_data_after_load(controller):
+    controller.loadFile("tests/fixtures/sample_stream.csv")
+    assert controller.graphSensorNames == "Sensor 1,Sensor 2,Sensor 3"
+    series = json.loads(controller.graphSeriesJson)
+    assert series == [
+        [149.0, 149.2, 149.1],
+        [148.5, 148.4, 148.6],
+        [150.6, 150.7, 150.5],
+    ]
+
+def test_graph_data_cleared_after_unload(controller):
+    controller.loadFile("tests/fixtures/sample_stream.csv")
+    controller.unloadFile()
+    assert controller.graphSensorNames == ""
+    assert controller.graphSeriesJson == "[]"
+
 def test_leaving_live_mode_blanks_wafer_map(controller):
     """Stopping a live session (tab switch → setMode) must not leave stale
     sensor values on the wafer map — it should read as blank."""